The present invention generally relates to call forwarding control systems, and more particularly to a call forwarding or call diversion control system for realizing a call forwarding function when connecting an integrated services digital network (ISDN) line and an ISDN terminal to a speech channel system.
When a private network made up of an ISDN is connected to a public network, there are cases where a call to the private network from a terminal of the public network is to be forwarding to another terminal of the public network. In such cases, it is desirable that the call forwarding function of the private network can be made without the need to use a new information channel of the ISDN line nor to make a switching control.
FIG. 1 shows an example of a conventional call forwarding control system. In FIG. 1, a main network 11 such as a public network is coupled to a private branch exchange (PBX) 14 via a T-point of an ISDN line. Terminals 12 and 13 of the public network 11 are coupled to the public network 11. A terminal 15 of the PBX 14 is coupled to the PBX 14 via a S-point of the ISDN line. The terminal 15 includes a call forwarding request part 21 for requesting a call forwarding at the terminal 15. In addition, the PBX 14 includes a call forwarding accept part 31 which accepts a call forwarding, and a call forwarding execution part 32 which executes the call forwarding.
For example, assume that the terminal 15 is called from the terminal 12 via the public network 11, an information channel 16 and the PBX 14, and that the terminal 15 is to forward this call to the other terminal 13 of the public network 11. In this case, the call forwarding request part 21 of the terminal 15 makes a call forwarding request to the PBX 14 by specifying a number of the terminal 13 as a forwarding destination number. The call forwarding accept part 31 of the PBX 14 accepts the call forwarding request and carries out a call control depending on the forwarding destination number. The call forwarding execution part 32 of the PBX 14 captures a new information channel 17 which is different from the information channel 16 to which the call is received from the terminal 12, and carries out a switching control using the new information channel 17. Hence, the call forwarding execution part 32 makes a call setup with respect to the terminal 13 via the public network 11 according to the forwarding destination number, so that a desired call forwarding is made by forming an information path between the called information channel and the new information channel 17.
According to the conventional call forwarding system for the private network, it is necessary to use a new information channel and to carry out a switching control when making a call forwarding to the other network, that is, the public network. Therefore, there were problems in that two information channels (ISDN lines) must be used at the T-point and that the private network must carry out the switching operation.
Next, a description will be given of the operation of the conventional call forwarding control system, by referring to the time charts of FIGS. 2 and 3. The time charts shown in FIGS. 2 and 3 show the procedure of the general point-to-multi-point connection interface.
First, if a user B of the public network 11 calls a user A at an extension of the PBX 14, a call setup (SETUP) is sent from the user B to the user A via the public network 11 and the PBX 14 as shown in FIG. 2. The call setup (SETUP) includes a call reference number (CR1), a bearer capability (BC) and a channel number (CH). The public network 11 returns a call proceeding (CALL PROC) to the user B in response to the call setup (SETUP). The PBX 14 returns a call proceeding (CALL PROC) to the public network 11 in response to the call setup (SETUP). In addition, the user A returns a call proceeding (CALL PROC) to the PBX 14 in response to the call setup (SETUP).
Second, the user A generates a call forwarding (CD1), that is, a call forwarding request. If a call forwarding is to be made to a user C, the user A sends an added information message (INFO) which includes a feature activation information element (FA[CD1]) and a keypad information element (KP[forwarding destination number]), so as to activate the call forwarding (CD1) with respect to the PBX 14. In FIGS. 2 and 3, a black circle indicates the inactive state in which the call forwarding feature lamp is OFF, and a pair of overlapping circles indicates executing state in which the call forwarding feature lamp slowly flashes. The user A sends the added information message (INFO) to the PBX 14 by adding the call reference number, the service request (FA[CD1]) of the call forwarding and the keypad number (KP), that is, the forwarding destination number. The PBX 14 sends a call setup (SETUP) to the public network 11 in response to the received added information message (INFO) from the user A, and the public network 11 sends the call setup (SETUP) to the user C.
Third, the PBX 14 sends to the user A a release message (REL) by adding the call reference number, the feature indication (FI) and the call forwarding display (DSP[DSP1]), where DSP denotes a display information element, DSP1 denotes a display information field, and the content of the display information field DSP1 is "starting forwarding", for example. The user A assumes the executing state (that is, slow flashing of the call forwarding feature lamp) in response to the release message (}{EL). In addition, the PBX 14 sends a release message (REL) to a user D who is other than the user A by adding the call reference number-and a disconnect/restoration cause (CAUSE [#26]) of the user who was not selected. The user A returns a release complete (REL COMP) to the PBX 14 by adding the call reference number. Further, the user D who is other than the user A also returns a release complete (REL COMP) to the PBX 14 by adding the call reference number.
Fourth, the user C sends an alert (ALERT) to the public network 11 as shown in FIG. 3, and the public network 11 sends an alert (ALERT) to the PBX 14. Hence, the PBX 14 returns an alert (ALERT) to the public network 11, and the public network 11 sends an alert (ALERT) to the user B. At the same time, the public network 11 sends a ring back tone (RBT) to the user B.
Fifth, if the user C lifts the receiver and a connection is made, a connect (CONN) is returned to the public network 11. Hence, the public network 11 sends a connect (CONN) to the PBX 14
Sixth, the public network 11 returns a connect acknowledge (CONN ACK) to the user C. On the other hand, the PBX 14 returns a connect acknowledge (CONN ACK) to the public network 11, and sends a connect (CONN) to the public network 11. In addition, the public network 11 sends a connect (CONN) to the user B to notify that the connection for the deflection has been made. The public network 11 then returns a connect acknowledge (CONN ACK) to the PBX 14, and the user B returns a connect acknowledge (CONN ACK) to the public network 11. Thereafter, a communication is made between the users B and C via the public network 11 and the PBX 14.
Therefore, in addition to the problems described above, it may be seen from FIGS. 2 and 3 that the conventional call forwarding control system provides no means for turning OFF the call forwarding feature lamp at the user A. Furthermore, the conventional call forwarding control system provides no means for notifying the user A upon the completion of the call forwarding operation.